The present invention generally relates to optical elements and more particularly, to an optical element incorporated in a laser beam scanning optical device in which a laser beam is deflected by a deflector in a plane so as to form an image on a photosensitive member.
Recently, various laser beam scanning optical devices for writing images have been proposed for use in a laser beam printer, a facsimile machine, etc. In these known optical devices, optical elements such as a cylindrical lens, a toroidal lens, a reflecting mirror, etc. are employed and are frequently made of optical plastic in place of hitherto used optical glass. If the optical elements are made of plastic, such advantages are achieved that the optical elements can be produced at low cost by mass production based on injection molding and can be formed into desired shapes easily. On the other hand, optical plastic has such drawbacks as high sensitivity to change of temperature and low rigidity. Therefore, the optical elements made of optical plastic are required to be prevented from being deformed due to change of shape with time or mechanical stress produced at the time of mounting of the optical elements.
Meanwhile, a transmitting surface or a reflecting surface of the optical elements such as a lens, a mirror, etc. is required to be machined to high surface roughness. Thus, in the case where the optical elements are made of plastic material, various structural restrictions are imposed on the optical elements. For example, in the case of a long reflecting mirror, when vertically and horizontally extending reinforcing ribs are provided rearwards of a reflecting portion having a reflecting surface so as to reinforce the reflecting mirror mechanically, the reflecting portion provided with the reinforcing ribs is subjected to shrinkage at the time of molding of the reflecting mirror, thereby resulting in aggravation of accuracy of the reflecting surface. Therefore, in the prior art reflecting mirror, it was impossible to provide a reinforcing means even rearwards of the reflecting surface.